Use clang 12.0.1 on dist-x86_64/i686-linux

The LLD + ThinLTO __morestack bug has been fixed in 12.0.1, so
we can now update our clang version. This also means that we no
longer need to build Python 2.

Author: nikic

src/ci/docker/host-x86_64/dist-i686-linux/Dockerfile

@@ -69,8 +69,6 @@ COPY host-x86_64/dist-x86_64-linux/build-gcc.sh /tmp/
 RUN ./build-gcc.sh && apt-get remove -y gcc g++
 
 COPY host-x86_64/dist-x86_64-linux/build-python.sh /tmp/
-# Build Python 2.7 needed for Clang 10.
-RUN ./build-python.sh 2.7.12
 # Build Python 3 needed for LLVM 12.
 RUN ./build-python.sh 3.9.1
 
@@ -97,7 +95,7 @@ ENV RUST_CONFIGURE_ARGS \
       --build=i686-unknown-linux-gnu \
       --set llvm.ninja=false \
       --set rust.jemalloc
-ENV SCRIPT python2.7 ../x.py dist --build $HOSTS --host $HOSTS --target $HOSTS
+ENV SCRIPT python3 ../x.py dist --build $HOSTS --host $HOSTS --target $HOSTS
 ENV CARGO_TARGET_I686_UNKNOWN_LINUX_GNU_LINKER=clang
 
 # This was added when we switched from gcc to clang. It's not clear why this is

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -69,8 +69,6 @@ COPY host-x86_64/dist-x86_64-linux/build-gcc.sh /tmp/
 RUN ./build-gcc.sh && apt-get remove -y gcc g++
 
 COPY host-x86_64/dist-x86_64-linux/build-python.sh /tmp/
-# Build Python 2.7 needed for Clang 10.
-RUN ./build-python.sh 2.7.12
 # Build Python 3 needed for LLVM 12.
 RUN ./build-python.sh 3.9.1
 
@@ -102,7 +100,7 @@ ENV RUST_CONFIGURE_ARGS \
       --set llvm.thin-lto=true \
       --set llvm.ninja=false \
       --set rust.jemalloc
-ENV SCRIPT ../src/ci/pgo.sh python2.7 ../x.py dist \
+ENV SCRIPT ../src/ci/pgo.sh python3 ../x.py dist \
     --host $HOSTS --target $HOSTS \
     --include-default-paths \
     src/tools/build-manifest

src/ci/docker/host-x86_64/dist-x86_64-linux/build-clang.sh

@@ -4,7 +4,7 @@ set -ex
 
 source shared.sh
 
-LLVM=llvmorg-10.0.0
+LLVM=llvmorg-12.0.1
 
 mkdir llvm-project
 cd llvm-project

src/ci/pgo.sh

@@ -4,7 +4,7 @@ set -euxo pipefail
 
 rm -rf /tmp/rustc-pgo
 
-python2.7 ../x.py build --target=$PGO_HOST --host=$PGO_HOST \
+python3 ../x.py build --target=$PGO_HOST --host=$PGO_HOST \
     --stage 2 library/std --rust-profile-generate=/tmp/rustc-pgo
 
 RUSTC_BOOTSTRAP=1 ./build/$PGO_HOST/stage2/bin/rustc --edition=2018 \